1. Field of the Invention
The present invention relates to a method for preserving plywood and particle board against undesirable reactions caused by microorganisms, particularly those caused by fungi and moulds, resulting in decay, mould and blue-stain deterioration of wood.
According to the present method, a wood composite product is made from separate wood pieces by joining said pieces together with an adhesive or binder. The wood pieces may comprise planar wood veneer sheets or smaller wood particles (chips, sawdust or wood powder). The preserving additive possessing growth- and spreading-inhibiting properties against microorganisms is mixed into the base adhesive used in the method.
The invention also concerns an adhesive composition with particularly advantageous properties for producing plywood and particle boards, as well as methods for preparing such an adhesive composition.
2. Description of Related Art
Rot fungi and other microorganisms use the structural components of wood in their metabolism or degrade wood by the growth of their colonies. Wood degradation, particularly in the form of an essential loss of its strength properties, is chiefly caused by rot fungi, of which brown-rot, white-rot and cubical dry-rot fungi deserve mentioning. Further, the species of these fungi causing the greatest damages are those of the brown-rot fungi including dry-rot fungus (Serpula lacrymans), cellar fungus (Coniophora puteana) and white-pore fungi (Poria sp., Antrodia sp., Fibroporia sp.). Brown-rot fungi decompose the polysaccharides of wood causing rapid deterioration of wood strength properties already during the initial state of rot, even prior to a detectable weight loss of the wood.
Damage to wood is also caused by blue-stain and mold fungi mostly causing discolouration defects. The blue-stain fungi may in some cases bring about a minor weight loss in the wood, however, generally less than 3%. When occurring in great abundance, blue-stain fungi modify the permeability properties of wood, whereby wood becomes more permeable to water, whereby favourable growth conditions may be formed for the actual rot fungi.
The most common blue-stain fungi found in wood are those of the Ambrosiella, Aureo-basidium, Ceratocystis, Cladosporium, and Phialosphora families. Blue-stain fungi most frequently occurring in pine wood material are Aureobasidium pullulans, and the species of the Ceratocystis family such as C. pilifera. In addition to these, blue-staining of sawn spruce is caused by such species as, e.g., Ceratocystis piceae and C. coerulens. In addition to the fungi of the families mentioned above, blue-staining of sawn pine is also caused by species of the Sclerophoma family such as Sclerophoma entoxylina. Of fungi causing mould damage, the most important to mention are species of the Cladosporium, Altenaria, Helminthosporium, Penicillium, Aspergillus, Epicoccus and Rhizopus families. Mold fungi belonging particularly to the Penicillium and Aspergillus families cause extensive damage in indoor spaces and structures.
To preserve wood against decay and damage caused by microorganisms, different types of methods and preservative compositions have been developed. The most common method is to subject wood to pressure impregnation with preservatives (e.g., salt preservatives and creosote oil), which are capable of preventing growth of microorganisms in wood. With plywood and other similar wood composite materials, the most commonly used preservation treatment comprises admixing the preservative into the adhesive during the manufacture of the composite sheet. Typically, the active preservative compound to be admixed into the adhesive is selected from the group of chlorinated phenols, organic tin compounds, fluorides, and today, Xyligen, moraside and xylasane compounds. Besides using a preservative in the adhesive, wood composite products may be protected by spraying the wood veneer sheets or chips with a preservative solution. Veneer sheets may also be treated by immersion. Ready-laminated sheets can be protected by pressure or vacuum impregnation, whereby the preservatives are selected from the groups of salt preservatives and oil-based preservative formulas containing organic tin compounds.
A disadvantage of preservatives used for impregnating sawn goods is their general toxicity necessitating the handling of such preservative residues and wood blocks treated with them as hazardous waste. Also the other above-mentioned preservative compositions used for preserving wood composite products are handicapped by the same problem.
FI Patent No. 90,951 discloses a wood preservation method capable of overcoming the problems associated with conventional preservation of sawn wood material. In the method disclosed therein, complexing agents are used to bind in the wood into chelates at least a portion of the metals, which are essential to the metabolism of the decaying microorganisms. Preservation of wood in the method is carried out in a conventional manner by impregnating the wood being treated with a solution of the complexing agent.
Up to now, the use of complexing agents in the preservation of plywood and similar wood composite products has not been considered in the art. In particular, the preservation of plywood and particle board or similar wood composite products made from wood pieces/particles and a binder used to adhere such pieces/particles together against decay and moulds by means of admixing complexing agents into the binder used in the fabrication of said products during the manufacturing process for the specific purpose of improving the resistance of the wood product to deterioration and decay due to fungi and mold has not been considered. According to the prior knowledge of the art, this has not been considered possible, because the adhesives used in the process are two-component formulations, in which the hardeners contain metals capable of reacting with the complexing agents. For instance, the hardener of conventional plywood bonding adhesive contains, among other components, wood bark extract and chalk, whereby large amounts of heavy metals and other metallic compounds are contained therein. On the basis of such knowledge, a conclusion has been made in the art that the admixture of complexing agents into the adhesive of plywood and similar materials would decisively deteriorate the properties of the adhesive resulting in the precipitation, change of pH and inhibition of gel formation in the adhesive.
It is an object of the present invention to overcome the drawbacks of the above-described prior-art techniques and to provide an entirely novel method for preserving plywood, particle board and similar wood composites against rot, mould and other similar decay and quality-degrading reactions.
It is another object of the present invention to provide a wood composite product which is protected against rot, mould and other similar decay and quality-degrading reactions.
It is a third object of the present invention to provide a binder composition for use in the manufacture of wood composite products, and it is still a further object of the present invention to provide a method of producing a binder suited for use in the manufacture of plywood, particle board and similar wood composite products.
These and other objects of the invention are achieved through preserving the wood composite against the undesirable reactions caused by the above-mentioned microorganisms by admixing into the adhesive or binder compound used in the manufacture of plywood and particle board such complexing agents that are capable of forming chelates with metals essential to the rot mechanisms and general metabolism of fungi. In fact, the invention is based on an unexpected discovery that, contrary to a general belief, a chelate-forming agent admixed into the adhesive can migrate from the adhesive due to the moisture released from the adhesive and the processing conditions of sheet products into the veneer sheet or chips of the product without causing any significant changes in the composition or properties of the adhesive. By contrast, it has been found that complexing agents can accelerate the cure of a phenolic resin adhesive in plywood manufacture and, when admixed into the melamine urea formaldehyde resin used in particle board bonding, such complexing agents may even improve the cross-sheet tensional strength of the produced boards.
In the wood material of plywood and particle board, the chelating compound forms complex compounds with the transition group metals, principally iron and manganese, and other metals essential to rot fungi and other similar microorganisms, thereby essentially improving the resistance of a wood composite product to rot and fungi.
According to the invention, the complexing agent is particularly advantageously admixed into the adhesive or binder already during the blending thereof. Thus, when a low-solubility complexing agent such as, e.g., divalent sodium salt of EDTA is used, the heat released by the exothermic condensation reaction of the binder can be utilized to improve the solubility of the complexing agent.
Besides conventional cross-ply laminated plywood and particle board, different types of lumber-core plywood and other boards and structural components formed from wood particles by adhesive bonding will be included under the term xe2x80x9cwood composite productsxe2x80x9d as used herein. Hence, the method may also be used in the preservation of, e.g., beams made from strip-like particles of veneer. The wood particles may be of coniferous or deciduous wood, or a combination thereof
In the context of the present invention, the term xe2x80x9cundesirable reactionsxe2x80x9d of microorganisms refers principally to deterioration and decay of wood due to fungi and moulds. The deterioration of wood, that is, an essential loss of its strength properties, is chiefly caused by rot fungi which include the brown-rot and white-rot fungi mentioned above. Damage to wood (in the form of discolouration defects) is caused, among other microorganisms, by blue-stain and mold fungi listed above. However, the weight loss caused by them is insignificant.
The invention can be used for preserving wood against undesirable reactions caused by any of the above-mentioned microorganisms.
In the context of the present invention, the term xe2x80x9ccomplexing agentxe2x80x9d (or xe2x80x9cchelating agentxe2x80x9d) refers to a compound capable of binding di- or trivalent cations into insoluble or soluble complex compounds.
Complexing agents used for binding metals are chiefly water-soluble, which makes them excellently suited for admixing into an adhesive. The adhesive primarily used for bonding of veneer sheets is phenolic resin. Bonding of particle boards intended for use under damp conditions is made using urea melamine resin.
Complexing agents can be divided into the groups of inorganic and organic compounds. The group of inorganic complexing agents consists of different cyclic and linear phosphate compounds including polyphosphates such as sodium polyphosphate (Na5P3O10, STPP). The most important organic complexing agents are amino carboxylic acids and their salts in which the acid part is formed by acetic acid, whereby examples of such agents include ethylenediamine tetraacetic acid (EDTA), n-hydroxyethylethylenediamine triacetic acid (HEDTA), diethylenetriamine pentaacetic acid (DTPA), nitrilo triacetic acid (NTA), ethylenediamine-di-(o-hydroxyphenyl acetic acid) (EDDHDA), diethanol glycine (DEG) and ethanol glycine (EDG) as well as salts thereof, particularly their alkali metal salts; hydroxy acids including gluconic acid, glucoheptonic acid and other sugar acids such as xcex2-glucoisosaccharic acid, xcex1-isosaccharic acid, tartaric acid, malic acid and citric acid as well as salts thereof; and organophosphates in which the acid part is formed by phosphoric acid, whereby examples of such agents include aminotrimethylene phosphonic acid (ATMP), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), ethylenediaminetetramethylene phosphonic acid (EDTMP), diethylenetriaminepentamethylene phosphonic acid (DTPMP) and salts thereof.
The invention can also be implemented using metal-binding phenolates or catecholates such as the biological chelating agents, known as siderophores, which are produced by microorganisms. Siderophores are complexing agents excreted by microorganisms for the purpose of binding metal ions, particularly iron, from the substrate for the metabolism of the organism. The siderophores produced by some fungi (Trichoderma sp.) and bacteria (Pseudomonas sp.) have been found to possess an inhibiting function to the growth of other microorganisms, based on the strong affinity of the siderophores for the iron contained in the growth substrate.
According to a preferred embodiment of the invention, the complexing agent used is aminocarboxylate or aminocarboxylic acid, most appropriately aminotetracarboxylates or aminotetracarboxylic acids. In the examples to be described later, ethylenediamine tetraacetic acid (EDTA) and its salts (ethylenediamine tetraacetates) are used with a particularly high efficiency in the method according to the invention. Suitable salts of EDTA are its alkali metal salts, particularly the sodium salts, Na2-EDTAc and Na4-EDTAc.
According to the invention, into the adhesive or binder used in the manufacture of plywood and particle board is admixed such an efficient amount of a complexing agent that is sufficient for achieving preservation of the wood material. Along with the adhesive, the preservative is transferred into the wood veneer sheets or chips of the particle board so that a maximally large proportion of the metals natively contained in the wood can be bound into a form unsuitable for participating in the general metabolic reactions of fungi or catalyzing the reactions of the decay process. In practice, at least approx. 10%, advantageously over approx. 20% of all the above-mentioned metals contained in the wood are bound.
The properties of the complexing agent, particularly its solubility as well as the solvent of the complexing agent and the binder used are the factors that determine the admixing temperature of the complexing agent. Advantageously, the admixing step is carried out at about 5xc2x0 C. to about 110xc2x0 C. Typically, the operation is carried out at room temperature from about 15xc2x0 C. to about 25xc2x0 C. For low-solubility complexing agents, an elevated temperature is preferred. Thus, the admixing of, e.g., Na2-EDTA into the adhesive is preferably made at a temperature of at least 30xc2x0 C., particularly advantageously at about 40-110xc2x0 C. (Under ambient pressure conditions, the upper temperature limit is about 95xc2x0 C.)
According to a preferred embodiment of the invention, the complexing agent is admixed into the adhesive composition already during its preparation. Admixing may be made in conjunction with the polymerization of the polymer resin acting as the binder and/or immediately thereafter. Thence, when preparing a phenol formaldehyde based resin, the complexing agent (or a part thereof) can be metered with the phenol-formaldehyde resin, as well as the catalyst possibly used therein, in the condensation reactor, whereby the complexing agent is dissolved in the aqueous phase of the reaction mixture (typically containing 40-60% water) under the heat released by the condensation reaction. Typically, the reaction temperature in the reactor reaches approx. 70-110xc2x0 C. (under ambient pressure conditions maximally approx. 95xc2x0 C.), whereby the solubility of, e.g., Na2-EDTA and similar low-solubility complexing agents into the adhesive composition is increased manifold. Another alternative is to admix the complexing agent into the resin mixture immediately after the completion of the condensation reaction before the cooling thereof is started.
The catalyst in the phenol-formaldehyde condensation reaction typically is an acid (e.g., HCI), a base (e.g., NaOH) or an amine. Because many of the suitable complexing agents also contain amine groups, the catalyst may frequently be replaced by the complexing agent in the blending of the binder.
In proportion to the amount of binder resin used, into the raw material mixture is admixed the complexing agent by approx. 1-30 parts per weight and the other starting materials (including water contained therein) by approx. 99-70 parts per weight, respectively, to obtain an adhesive composition in which the amount of the complexing agent is approx. 1-30 wt-%, advantageously approx. 2-20 wt-%.
Besides for phenol formaldehyde resin adhesives, the above-described arrangement is generally applicable to the preparation of such adhesive compositions which contain a resin producing heat during its polymerization reaction or which are polymerized by heating the mixture of starting materials to a temperature of at least approx. 40xc2x0 C.
As noted above, the amount of the complexing agent may be varied widely in the adhesive composition. Typically, the goal is to admix into the adhesive or binder so much of the preservative that its amount in the end product is approx. 1-30 kg/m3 wood, advantageously approx. 5-20 kg/m3 wood.
Complexing agents can be used particularly advantageously admixed into two-component adhesives such as phenolic resin binders and melamine urea formaldehyde resins. Most appropriately complexing agents are used in conjunction with such adhesives that are miscible with polar solvents such as water and after drying/curing become water-insoluble. Thence, particle boards for use under damp conditions can be made by using binders in which complexing agents and melamine urea formaldehyde resins are combined.
The adhesive containing the complexing agent may be brought onto the surface of the wood pieces/particles being processed with the help of conventional glue application techniques such as spraying, or in conjunction with the lay-up of sheets, using brush/roll application.
Water is advantageously used as the solvent or dispersant of the preservative, and the preservative composition may also contain other conventional admixtures capable of promoting the penetration of the solution or dispersion into the wood. Besides biologically inert admixtures, the preservative composition according to the invention can contain conventional biologically active compounds such as copper ions or complex compounds of copper.
The invention provides significant benefits. Thence, as mentioned above, the wood preservative composition according to the invention is water-soluble and miscible in an adhesive or binder, which makes it environmentally compatible in this respect. Further, the preservative does not contain any substances of general toxicity, but rather is highly specific to wood-decaying microorganisms, particularly fungi, occurring in wood. The method according to the invention utilizes efficiently the capability of chemical complexing agents and siderophores produced with the help of microorganisms in binding iron, other metals of the transition group and bioactive compounds contained in the substrate formed by the product so as to prevent the growth and colonization of fungi.
A further unexpected discovery has been made therein that a complexing agent admixed into the phenolic resin binder of a plywood product can speed the cure of the binder and that the strength properties of particle boards made using melamine urea formaldehyde resin (MUF) as the binder are improved if a complexing agent is admixed into the binder. Additionally, tests performed on faced plywood have shown that a complexing agent can give a better protection against decay by white rot than the conventionally used preservative (Xyligen).